I'm getting the same thing with both of my Echo Dots. I don't have an Echo, so I can't check if it works on that.
Strangely I have got the odd one or two to work, here are some I tried:
"Alexa, What's nineteen plus five" => "The sum of 19 and 5 is 24."
"Alexa, What's seventeen plus five?" => "Hmm, I'm not sure."
"Alexa, what's seventeen plus fifteen?" => "Hmm, I'm not sure."
"Alexa, what's seventeen plus six?" => "The sum of 17 and 6 is 23."
"Alexa, what's sixteen plus seven?" => "Hmm, I don't know that one."
I copied the requests and responses from to history, so it definitely recognised what I said.
Also, I have tried these examples multiple times and it is always the same ones which work and don't work so it doesn't appear to be a random misrecognition.